The proposed research uses magnetic resonance spectroscopy (MRS) to explore the link between cardiac performance and physiochemical regulation in postischemic hearts. The general problem to be addressed is the impaired contractile function of myocardium that is reperfused after non-lethal ischemia. The experimental design focuses on the regulated entry of glycolytic end products into the mitochondria in support of the energetic demands of cardiac function. An approach is proposed to monitor perturbations of the balance between the cytosolic and mitochondrial distribution of intermediates in the isolated rabbit heart using MRS. Although the pathogenesis of postischemic contractile dysfunction is known to be multifactorial, work by the applicant and others has suggested a significant link between postischemic contractile recovery and activation of the key mitochondrial enzyme pyruvate dehydrogenase (PDH). This enzyme regulates the balance between the oxidative and nonoxidative fates of glycolytic end products and coordinates the relative distribution of intermediates between the mitochondrial and cytosolic compartments. Targeting PDH with carbon-13 (13C) labeled substrates has allowed shifts in this oxidative/nonoxidative distribution of metabolites to be discerned in the intact heart with MRS. Thus, the use of 13C MRS provides an experimental approach to enable on-line observations of the physiochemical balance between subcellular compartments. Measures of cardiac performance along with 13C MRS of postischemic hearts show that metabolic reversal of postischemic contractile dysfunction can be achieved by stimulating PDH. This recovery appears related to increased oxidation of glycolytic end products, although an alternative mechanism may involve activation of the branched chain keto-acid dehydrogenase. This proposal explores potential mechanisms by which enzyme activation influences contractile recovery during reperfusion. Hearts will be supplied 13C enriched substrates to probe enzymatic activity in testing a three-fold hypothesis that: 1) countering depressed PDH activity at reperfusion avoids the production of lactate which otherwise impairs recovery via increased energetic demands due to proton load; 2) activating the branched chain keto-acid dehydrogenase promotes recovery of oxidative metabolism in support of contractility; 3) 13C NMR of intact hearts reflects changes in the balance of cytosolic and mitochondrial metabolites. Experiments explore regulatory mechanisms of enzyme activity that support the recovery of postischemic hearts.